This invention generally relates to decoding circuits and more particularly to a decoding circuit suitable for a differential Manchester code.
Conventionally, the type of code for data transmission has been studied by paying attention to how much binary logical values "1" and "0" can be transmitted and received correctly. However, as communication systems such as local area networks have advanced and consequently, increasing demands for information transfer in the form of packet have arisen, a code has been sought which can be adapted for transmission of ternary and quaternary data instead of the conventional binary data. Such a code is typically exemplified by the differential Manchester code. Details of the differential Manchester code are discussed in Draft-IEEE Standard 802.5 by IEEE PROJECT 802 entitled "Token Ring Access Method and Physical Layer Specifications", Working Draft, Dec. 1, 1983.
According to this publication, the differential Manchester code has quaternary symbols of "1", "0", "J" and "K" which are coded by using two signal elements for each symbol. To be specific, the symbol "1" does not make an inversion at the boundary of that symbol but makes an inversion in the middle of that symbol. The symbol "0" makes inversions at the boundary of and in the middle of that symbol. The symbol "J" makes no inversion at the boundary and in the middle of that symbol. The symbol "K" makes an inversion at the boundary of that symbol, but does not make an inversion in the middle of that symbol. Accordingly, the coding rule of the differential Manchester code is based on whether transition (inversion) of the polarity takes place at the boundary between the signal elements.
As will be seen from the above explanation,
(1) The symbols "0" and "1" make a transition in the middle of those symbols; and
(2) The symbols "0" and "K" make a transition at the boundary adjacent the preceding symbol.
Therefore, detection of the symbol boundary is indispensable for decoding the differential Manchester code. However, this type of code does not involve a sole specified pattern suitable for the detection of the symbol boundary. Therefore, in a decoding circuit of the differential Manchester code, a method has been available which uses combinations of a plurality of symbols for the detection of the symbol boundary in accordance with a protocol of data transmission for which the differential Manchester code is used. According to a protocol described in the above-mentioned Working Draft by IEEE PROJECT 802, for example, a frame and a token start from consecutive symbols of "J, K, 0, J, K, 0, 0, 0" (hereinafter referred to as starting delimiter "SD") and terminate in consecutive symbols of "J, K, 1, J, K, 1" (hereinafter referred to as ending delimiter "ED"). Information to be transmitted is inserted between SD and ED. The conventionally known decoding circuit follows the aforementioned protocol and uses specified patterns (SD and ED) prescribed by this protocol for detecting and decoding the symbol boundary, thus facing problems as below.
(1) The decoding circuit must be re-designed according to protocols prescribing different SDs, EDs and other special patterns.
(2) Delay time in the decoding circuit is large.
(3) Amounts of hardwave required for the decoding circuit are large.